Remove inbound request signature verification (closes #279)

The receiver verified an optional per-entrypoint HMAC or shared token
before accepting a request. That is removed outright: the entrypoint
UUID in the URL is the authentication secret, and possession of it
authorises submission. This reverses the feature added in fcead5d.

Deletes the internal/signature package, the signature_scheme and
signature_secret columns from Entrypoint along with their accessors,
the per-entrypoint secret form and its POST route, and the scheme
labelling in EntrypointView. Pre-1.0 with no installed base, so the
columns simply stop being written; there is no migration and no
compatibility path.

Header sanitisation goes with it. SanitizeHeaders existed to strip a
scheme's own credential header before the header map was stored and
forwarded; with no configured credential there is nothing to strip, so
the receiver marshals the headers as received.

The receiver's other protections are untouched: the 1 MB body cap, the
per-IP rate limiter, the 410 for a deactivated entrypoint and the 404
for an unknown UUID.

internal/server/routes.go

@@ -237,10 +237,6 @@ func (s *Server) setupSourceRoutes() {
 			"/entrypoints/{entrypointID}/toggle",
 			s.h.HandleEntrypointToggle(),
 		)
-		r.Post(
-			"/entrypoints/{entrypointID}/secret",
-			s.h.HandleEntrypointSecret(),
-		)
 		r.Post("/targets", s.h.HandleTargetCreate())
 		// The edit form is the one page that renders a target's
 		// destination URL and header values in full; see